It seems the prep cost of locker items is generally double the price you bought it for in the shop (if it's a shop item). The Tinker buys everything at a discount of 5 gold, so if you use the Tinker to hunt for locker items, your locker preps become cheaper by 10 gold. If the Tinker acquires the item through a Translocation scroll, it doesn't become a free prep but it still gets the Tinker price discount.

What happens to the costs when you exit the game and start it back up again? I'm surprised that locker items have varying cost information - that's actually never stored anywhere for them AFAIK. Backpack items maybe...